Gene targeted mice with a simpler immune system. This grant application proposes to investigate the influence of endogenous retroelements on autoimmune disease. Almost half of our genome consists of retroelements-many of them active. We have three lines of evidence for our hypothesis that retroelements can mediate autoimmune disease: (i) The enzyme Trex1 degrades retroelements, and Trex1-deficient patients and mice suffer from autoimmune disease. (ii) Similarly, an integrase inhibitor that accumulates endogenous retroelement cDNA exacerbates lupus and hemolytic anemia in mice. (iii) Apart from its function as a DNA mutator, AID inhibits LINE-1 elements, and AID-deficient patients develop a variety of autoimmune or inflammatory disorders. In this proposal, we will inhibit retroelement replication by transgenesis, and the mice carrying the transgenes ought to have no or ameliorated autoimmune disease.